Light conduits, also known as light pipes, formed of structured surface materials and operating on the basis of total internal reflection are well-known. Such light conduits are described in U.S. Pat. No. 4,260,220 (Whitehead). As described in that patent, the light conduit is used primarily for transporting light. Alternatively, such a light conduit may be used as a lighting fixture or luminaire as described in U.S. Pat. No. 4,615,579 (Whitehead). Such a luminaire has the advantage that it provides even illumination over an extended region.
All of the light conduits described in the Whitehead references mentioned above, have flat sides. Experimentation has shown, however, that the transport efficiency of a light conduit having curved sides, and in particular a round cross-section, is, in general, better than that of such a pipe having flat sides. Such a light pipe is described in U.S. Pat. No. 4,805,984 (Cobb).
It is often desirable to use an extractor to help force light out of the tube forming the light conduit. This is particularly true when the light conduit is to be used as a luminaire. U.S. Pat. No. 4,850,665 (Whitehead) describes such an extractor. A preferred material for the extractor of that patent is a nonwoven polyester fiber formed into a sheet such as that sold under the registered trademark Tyvek by E. I. DuPont de Nemours and Company.
There are two related disadvantages that arise when light conduits such as those described in the above referenced patents are used as luminaires. Both problems result from the fact that these systems make no attempt to control the angle at which light escaping the tube strikes the smooth inner surface of the tube. The first problem relates to the efficiency of luminaires such as those described in the references discussed above. Experimentation and computer simulations have shown that light escapes the wall of the tube most efficiently when the projection of the direction of travel of the light ray in the plane perpendicular to the axis of the light pipe forms an angle of approximately 60 degrees with the normal to the smooth inner surface of the tube. In this context the phrase "most efficient" means that the highest percentage of the light striking the surface is transmitted and escapes from the light pipe. Since the prior art luminaires do not control the angle at which the light strikes the tube, they do not insure maximum efficiency.
The second problem is closely related to the first. Since the prior art systems do not control the angles at which the light strikes the inner surface of the tube, they tend to produce diffuse or otherwise uncontrolled light output. While this works well for some circumstances, it is not acceptable in situations where a strongly directed beam of light is desired.